The invention relates to a microwave system for the deicing of front areas of hollow spaces forming shell structure which are exposed to air flows and, as a result, subjected to meteorological influences and which are therefore subject to icing.
The formation of ice on such structures detrimentally affects the air flow around the structures which results, particularly in aeronautics, to problematic aerodynamic behavior.
Many efforts have been made to keep the front edges of such structures, which are at the greatest risk of icing, free of ice. The exposed surfaces of the front areas of such surfaces are for example sprayed or flushed with liquids which prevent the formation of ice, hot air is conducted across the inner surface areas or the areas are heated electrically by resistance heating systems. De-icing by liquids is limited by the liquid reservoir required and, furthermore, is considered to be unreliable.
It is necessary to suppress the conditions under which ice can form on the respective surfaces. This is possible with liquids only for a limited time, particularly with the use of deicing liquids on the ground before the start. The anti-icing film is torn off already during the starting phase and provides during the passage of an airplane through cloud formations in which the surfaces are subject to icing, only a relatively small time safety window. Rain washes such an anti-icing film off already on the ground relatively soon.
In aeronautics, it is common practice to blow hot air taken from the engines at the inner surfaces of the wings or, respectively, the aerodynamically important slats, that is the exposed front surfaces, particularly the wing tips. The heat transfer to the slats depends on the thermodynamic flow conditions and the meteorological circumstances and also on the travel height, the outside temperature, the travel speed, the droplet size, the lateral cloud formation, the water content of the air etc. Taking these parameters into consideration, the efficiency of a hot air anti-icing system is estimated to be about 30-40%.
Such a system results in a high power consumption and also in high losses in the supply ducts to the endangered areas of the airplane. In aeronautics, particularly in connection with modern engine technology, there are furthermore limits to the removal of sufficient amounts of hot air from the bypass flow of the engines so that it is not always possible to withdraw a sufficient amount of hot air.
In another technique, metallic nets or heating mats are disposed in the wall or on the inside wall of such structures which nets or mats can be electrically heated so that, by resistance heating, the respective surface areas can be heated or kept warm as desired. Because of the high power requirements, the electric supply lines from the onboard generator to the connecting points of the nets or heating mats, have to have a large cross-section. A homogeneous heating, that is avoiding excessive local heating, particularly in the area of the contact bars is always problematic when electric power is to be supplied to an extended area and must be carefully observed. In addition, the heat transfer to the problem areas is generally difficult.
DE 197 45 621 C1 discloses a de-icing procedure wherein a thin layer with hydrophobic properties of diamond-like carbon/amorphous hydrocarbon is deposited on the surfaces to be de-iced and, upon the formation of ice, the surface areas are irradiated by an outer infrared radiation source or are heated by a heating mat which is in contact with the surface areas and are excited and heated thereby.
DE 197 50 198 C2 discloses a technique for de-icing airplanes by microwaves wherein the microwaves are fed to the areas to be de-iced from a remote source disposed in the airplane fuselage. Fluid dynamically important areas of an airplane, which are sensitive to icing, consist of compound materials whose dielectric areas are permeable for microwaves above 20 GHz. For conducting the microwaves, suitable hollow conductors comparable to present hot air pipes, extend in the airplane fuselage within the wings from the microwave generator up to those areas where the microwaves are then uncoupled and keep these areas free of ice by heating the dielectric areas. Ice already formed is rapidly removed by heating of the interface area of the ice and the surface on which the ice has formed.
In lightweight body construction, increasingly hollow body or shell structures including pre-formed, CFK and GFK composite components are used. Although such composite materials are very form-stable and rigid and have a high mechanical strength in comparison with metal, they have, in comparison with metal, a relatively low an-isotropic thermal conductivity. As a result, heat can build up and the structure may overheat whereby local delaminations may occur when they are exposed to hot air. Concerning the flight safety the capability of supplying a sufficient power density to the surface area adjacent the air flow, which surface area is potentially coated with ice, is highly limited.
It is the object of the present invention to provide a compact de-centralized de-icing system for hollow or shell body structures which are exposed to atmospheric air flow and which are therefore subject to the formation of ice thereon.
In a microwave de-icing system for the front areas of exposed shell structures, at least one independently operable microwave generator is disposed in each shell structure closely adjacent the surfaces of the shell structure to be de-iced or kept free of ice and uncoupling means are flanged to the microwave generators with uncoupling openings disposed along the area of the shell structure to be heated so as to provide a microwave wave front directed toward this area. The area subjected to the wavefront includes a wall of a dielectric composite material with a metallic skin whereby the microwave front penetrates the wall and at least partially is converted into heat within the wall of the composite material thereby providing for a rapid and effective heat supply to the wall area of the shell structure to be kept free of ice.
To this end, a microwave source whose power output is controllable by way of pulse width control is disposed in the interior of the hollow or shell body structure and an uncoupling arrangement is flanged to the exit of the microwave source directly behind, or as close as possible behind, the front area which, on the outside, may be subject to ice formation thereon or which his to be kept free of ice. The mechanically stable hollow or shell body structures consist of CFK materials or of GFK materials or of pre-preg compound materials or a composition thereof. The outer surface of the structure consists of a metal film or a metal skin; at least the aerodynamically exposed outer surface is covered by such a film or skin which is connected along the whole edge thereof with adjacent metallic structures/surfaces, so that these hollow or shell body structures are microwave or high-frequency tight and do not permit electromagnetic radiation to be radiated out into the ambient area.
By way of the uncoupling structure, the microwave radiation is directed onto the front whereby the irradiated compound material volume is heated. Within this material, after startup, a temperature gradient is established which becomes smaller toward the outer skin. The microwave radiates controllably up to such a power level that, on one hand, at each location of the irradiated compound material volume a temperature-based safety distance of between 35 and 75xc2x0 C. from the delaminating temperature of TDL≈130xc2x0 C. of the compound material can be maintained and, on the other hand, there is, at the interface with the metal skin, a thermal surface area power density of up to 46 kW/m2, whereby ice formed on the surface of the structure can be melted so that it is released from the surface and fully ripped off by the air flow.
The uncoupling structure of the uncoupling arrangement is a hollow conductor which is flanged to the microwave source or sources and which has uncoupling openings for forming the necessary microwave front. They have different sizes and different distances from one another so as to provide for a uniform uncoupling of power along the hollow conductor. The radiation characteristic is such that the phase fronts present along the wing contour are as much as possible uniform and their amplitudes provide the de-icing surface power as locally required. For example, the wing tip may require substantially higher area power densities (up to 60 k/m2) than the rearward areas whose requirements may be lower by the factor 10.
For the protection of the microwave source, the microwave source may be closed in its transient and blocking attenuation by way of circulators.
Depending on the desired maximum microwave power output, the microwave source is either a klystron or a magnetron or an Extended Interaction Oscillator (EIO).
The microwave power exits with little attenuation at the uncoupling structure and selectively heats the surrounding shell structure, which acts as a low-grade dissipative resonator. Therefore, the hollow, that is, wave conductor with uncoupling structure is made of an electrically well conducting metal or, if weight savings require it, of compound material which is surrounded by a microwave-tight metal net. The metal net may also be at the inside of the wave conductor.
Such a microwave-based de-icing system may be disposed in arrangements or structures of a ship, or a train or a motor vehicle or other hollow structures which have to be kept free of ice and which may encounter all kinds of meteorological conditions.
The importance of such a de-icing system in aeronautics with regard to safety is obvious. Airplanes and helicopters have to have an aerodynamically suitable shape, particularly for structures needed for providing lift and control such as the wings, the rudder, the elevators and the edges surrounding the air inlets of the engines.
In larger airplanes, which, in the lift-relevant area of the front ends of the wings, have so-called slats, a reliably operating de-icing system is absolutely necessary for flight safety.
Another important field of application is the energy generation by wind power plants, which have very large rotor blades and which are constantly exposed to ground weather conditions.
In order to prevent icing of the rotor blades, a microwave source is disposed in the hub of the bladed rotor. From there, a hollow conductor with uncoupling structures extends into each blade and to front areas, which may be subjected to icing. The microwave source is arranged directly at the area of action. Only power supply lines and control lines have to be brought to the microwave source. There is no need for long hollow guide structures or wave guides.
With the microwave de-icing system, flight safety is improved because of the fast operation of the de-or anti-icing system. During so-called routine anti-icing operation, the power requirements are relatively low. Furthermore, certain icing conditions can be controlled which cannot be handled with conventional systems.
If compound materials are used for the slats, a substantial weight reduction of more than 20% compared to today""s metal construction can be achieved. Besides the safety, which has first priority, a more economical use of the airplane because of weight reductions and savings in fuel is possible. In addition, on the ground, the use of de-icing liquids, which are not absolutely safe and detrimentally affect the environment, can be reduced or actually eliminated.
With microwave technology, the temperature of the laminated structures can be kept significantly lower than with conventional de-icing systems blowing hot air at the structures to be de-iced. This permits higher power applications per surface area for certain de-icing situations at the outer skin. In any case, an operation is possible in which the compound material and the structure are thermally not stressed. Even clear ice attachment situations, which cannot be handled by the systems existing today, can be controlled without the danger of overheating and a resulting delaminating of the compound material.
For the de-icing of the slats, metal guide tubes in the wings and Piccolo tube systems as they are used today for the distribution of the hot air to the surface areas to be heated are eliminated. This results in additional weight savings. Furthermore, the slats can be built as independent modules, which can be easily exchanged during servicing at the airport. This is a substantial advantage since it permits time saving repairs.
Also, if one of the systems fails, it affects only a single slat which does not cause substantially deterioration of the airplane performance since the individual other slot deicing systems are not affected by the failure of the one system and continue to operate independently (redundancy).
With the conventional system, upon failure of part of the system, warm air must be withdrawn from the system supplying the other wing which substantially reduces the effectiveness of the overall system.
It is pointed out that all of the electrical power input is converted 100% into de-icing energy, which is transmitted to the metallic slats.
Along the slat contour, very different area densities are required; the highest requirements are at the front edges of the wings or slats. In order to prevent so-called run back icing, that is, the re-attachment of ice sliding back from the front areas of the slats, also the rear areas of the slats must be heated. In accordance with the necessary distribution along the slat contour, an optimized hollow microwave conductor uncoupling structure with corresponding radiation distribution characteristics for appropriate area coverage is provided (optimal power adaptation to the slat geometry, see FIG. 4).
The system is closed with respect to microwaves and electromagnetically sealed. The CFK/composite material is surrounded by a protective metallic skin, which primarily serves as lightning protection. It also prevents the escape of microwaves from the slat system. The closed structure of the slatsxe2x80x94hot air requires discharge channelsxe2x80x94has also aerodynamically the advantage that essentially laminar flow conditions can be established at the interface area and disturbing turbulence formation can be prevented.
The microwave de-icing system can be operated by pulse width control so that icing can be avoided in a prophylactic manner from conditions with small heating requirements up to clear ice removal with the highest heating power requirements.
The microwave technical de-icing system operates without any losses; the power taken from the net and supplied to the uncoupling structure is completely converted to de-icing or anti-icing energy. The performance of such a system is even more apparent as, with icing already present, the ice can be released from the exposed surfaces within a short time by melting of the interface area.
The microwave de-icing system which consists of at least one of the units presented below, will be described in greater detail on the basis of the accompanying drawings: